Leaky lake sediment is major source of Yellowknife Bay arsenic

Contaminated sediments contribute a significant load of arsenic to water in Yellowknife Bay, according to a new study.

Although arsenic concentrations in the bay remain within drinking water guidelines, the research suggests that lake sediments – contaminated by past gold mining operations – could continue releasing arsenic for decades.

Yellowknife sits on the western shore of Great Slave Lake’s Yellowknife Bay, and the region’s gold mining history has contributed a lot of arsenic to the area. Mining operations, largely Giant Mine, deposited arsenic to the bay through tailings, mine site runoff and atmospheric sources from the gold roasting process.

These materials went into Yellowknife Bay as different minerals and have been buried over time, according to study co-author Mike Palmer, a research scientist with Aurora Research Institute. The research was described in March in the Journal of Hazardous Materials.

As those minerals dissolve, Palmer said, arsenic can move into surface water.

“We’ve known that this process happens, but we’ve never really had an estimate of just how much arsenic was getting re-mobilized or diffusing into the overlying water,” he said.

To answer that question, Palmer and his colleagues collected water and sediment at 20 locations throughout Yellowknife Bay, from sites near Giant Mine to areas close to Dettah.

Some of the sediment samples were collected as sediment cores – tubes of sediment and water that can be incubated to gauge how much arsenic is moving in or out of the sediment.

At most sites, Palmer and his colleagues found that arsenic was moving from the sediments to the overlying water. Concentrations of arsenic in sediment and rates of arsenic release were raised at locations closer to Giant Mine, Palmer said, “which kind-of makes sense.”

“The closer you are to Giant Mine, the closer you are to that point source of pollution,” he added.

Arsenic concentrations in sediment and rates of release were also higher in deeper water than shallow areas, he said.

Extrapolating the estimates at sampling locations to the larger bay reveals that the arsenic load coming from lake sediments is comparable to that coming from Baker Creek, said Palmer. Baker Creek drains the Giant Mine site and is typically thought of as a major source of arsenic reaching Yellowknife Bay.

However, Palmer stressed that the study does not highlight a human health issue.

Concentrations of arsenic in the water of Yellowknife Bay are quite low, Palmer said. In fact, the team reported arsenic concentrations to be below five micrograms per litre, which meets the World Health Organization’s drinking water standard.

Nonetheless, Palmer said the findings suggest that sediment is an important consideration when looking at the bay’s recovery.

“It’s not just about shutting off the inputs from the mine site itself,” he said. Even if arsenic coming from Giant Mine were reduced, he said, there would still be a similar source coming from the lake bottom.

In the future, Palmer said continued monitoring will be important to ensure that arsenic coming from lake sediments doesn’t increase. He said conditions that might promote the release of arsenic, such as extensive work on the lake bottom or dredging, should be avoided.

It’s also possible that the effects of climate change could alter the stability of arsenic in sediments, according to Palmer. He added that more work is needed to understand the processes that influence how arsenic moves to and from sediment, as well as whether these processes may be affected by climate impacts.

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This article is produced under a Creative Commons CC BY-ND 4.0 licence through the Wilfrid Laurier University Climate Change Journalism Fellowship.